Harvey J. M. Hou

Water Splitting by Tungsten Oxide Prepared by Atomic Layer Deposition and Decorated with an Oxygen-Evolving Catalyst†

When sunlight is used as direct energy input, water can be split into hydrogen and oxygen at conversion efficiencies similar to those of solar cells. This process offers a method for energy storage to address the problem that the sun does not shine continuously, and is a particularly appealing approach to solar-energy harvesting. Notwithstanding the intense research efforts, progress in this area is extremely slow. Efficient and inexpensive water splitting remains elusive. A key reason for the sluggish progress is the lack of suitable materials. The “ideal” material must absorb strongly in the visible range, be efficient in separating charges using the absorbed photons, and be effective in collecting and transporting charges for the chemical processes. Such a material has yet to be found. The difficulties in finding a suitable material stem from the competing nature of intrinsic material properties (e.g., optical depth, charge diffusion distance, and width of the depletion region, among others), which leaves limited opportunity for tunability. We recently demonstrated that heteronanostructures, a type of nanoscale material consisting of multiple components that complement each other, have a combination of properties which are not available in singlecomponent materials. For instance, we can add chargetransport components to oxide semiconductors to solve the issue of low conductivity that oxide semiconductors generally suffer. In a similar fashion, one can add an effective catalyst to address the challenge that oxygen evolution is complex and tends to be the rate-limiting step. These new materials will likely lead to significant improvement in solar watersplitting efficiencies. The success of a heteronanostructure design relies on the ability to produce high-quality components with interfaces of low defect density, and on the availability of various components. Here we show that crystalline WO3 can be synthesized by the atomic layer deposition (ALD) method in the true ALD regime. When coated with a novel Mn-based catalyst, the resulting WO3 survives soaking in H2O at pH 7 and produces oxygen by splitting H2O under illumination. We choose ALD to prepare WO3 because of the following advantages: 1) a high degree of control over the resulting materials; 2) excellent step coverage to yield conformal coatings; and 3) process versatility to tailor the composition of the deposit. WO3 was studied because it is one of the most researched compounds for water splitting. The widely available literature makes it easy to compare our results with existing reports and thus allows us to test the power of the heteronanostructure design. To avoid the production of corrosive byproducts during the ALD process and to ensure the reaction occurs in the true ALD regime, we used (tBuN)2(Me2N)2W as tungsten precursor and H2O as oxygen precursor, as described in the Experimental Section (see Supporting Information for more details). Our first goal was to verify that the growth indeed takes place in the ALD regime. The dependence of the growth rate on the precursor pulse times and on the substrate temperature unambiguously confirms this. In addition, the excellent linear dependence of the deposition thickness on the number of precursor pulses supports the ALD growth mechanism and shows the extent of control we can achieve (see Supporting Information). That a long H2O pulse time is necessary to initiate growth is a key finding of this work. Despite intentional strengthening of the oxidative conditions, as-grown WO3 exhibited a tinted color, indicating the existence of oxygen deficiencies, which was then corrected by an annealing step in O2 at 550 8C. The crystalline nature of the product is manifested in the highresolution (HR) TEM image in Figure 1a. We also synthesized WO3 on two-dimensional TiSi2 nanonets. [18,19] The uniformity and good coverage around the nanonet branches show that this deposition technique is suitable for the creation of heteronanostructures. Ready dissolution of WO3 in aqueous solutions with pH 4 is a significant challenge that impedes its widespread use. We sought to solve this problem by coating WO3 with an Mnbased catalyst. Derived from the Brudvig–Crabtree catalyst, this coating was prepared by thermally decomposing [(H2O)(terpy)Mn(O)2Mn(H2O)(terpy)](NO3)3 (terpy= 2,2’:6’,2’’terpyridine). Similar to the oxo-bridged dimanganese catalyst, the thermal decomposition product exhibits good [*] R. Liu, Y. Lin, S. W. Sheehan, Prof. Dr. D. Wang Department of Chemistry, Merkert Chemistry Center Boston College 2609 Beacon St., Chestnut Hill, MA 02467 (USA) Fax: (+1)617-552-2705 E-mail: dunwei.wang@bc.edu Homepage: http://www2.bc.edu/~dwang

Structural and mechanistic aspects of Mn-oxo and co-based compounds in water oxidation catalysis and potential applications in solar fuel production.

To address the issues of energy crisis and global warming, novel renewable carbon-free or carbon-neutral energy sources must be identified and developed. A deeper understanding of photosynthesis is the key to provide a solid foundation to facilitate this transformation. To mimic the water oxidation of photosystem II oxygen evolving complex, Mn-oxo complexes and Co-phosphate catalytic material were discovered in solar energy storage. Building on these discoveries, recent advances in solar energy conversion showed a compelling working principle by combing the active Mn-oxo and Co-based catalysts in water splitting with semiconductor hetero-nanostructures for effective solar energy harnessing. In this review the appealing systems including Mn-oxo tetramer/Nafion, Mn-oxo dimer/TiO(2), Mn-oxo oligomer/WO(3), Co-Pi/Fe(2)O(3), and Co-Pi/ZnO are summarized and discussed. These accomplishments offer a promising framework and have a profound impact in the field of solar fuel production.

Manganese-based Materials Inspired by Photosynthesis for Water-Splitting

In nature, the water-splitting reaction via photosynthesis driven by sunlight in plants, algae, and cyanobacteria stores the vast solar energy and provides vital oxygen to life on earth. The recent advances in elucidating the structures and functions of natural photosynthesis has provided firm framework and solid foundation in applying the knowledge to transform the carbon-based energy to renewable solar energy into our energy systems. In this review, inspired by photosynthesis robust photo water-splitting systems using manganese-containing materials including Mn-terpy dimer/titanium oxide, Mn-oxo tetramer/Nafion, and Mn-terpy oligomer/tungsten oxide, in solar fuel production are summarized and evaluated. Potential problems and future endeavors are also discussed.

Bimodal intramolecular excitation energy transfer in a multichromophore photosynthetic model system: hybrid fusion proteins comprising natural phycobilin- and artificial chlorophyll-binding domains.

The phycobilisomes of cyanobacteria and red-algae are highly efficient peripheral light-harvesting complexes that capture and transfer light energy in a cascade of excitation energy transfer steps through multiple phycobilin chromophores to the chlorophylls of core photosystems. In this work, we focus on the last step of this process by constructing simple functional analogs of natural phycobilisome-photosystem complexes that are based on bichromophoric protein complexes comprising a phycobilin- and a chlorophyll- or porphyrin-binding domain. The former is based on ApcE(1-240), the N-terminal chromophore-binding domain of the phycobilisomes L(CM) core-membrane linker, and the latter on HP7, a de novo designed four-helix bundle protein that was originally planned as a high-affinity heme-binding protein, analogous to b-type cytochromes. We fused a modified HP7 protein sequence to ApcEΔ, a water-soluble fragment of ApcE(1-240) obtained by excising a putative hydrophobic loop sequence of residues 77-153. HP7 was fused either to the N- or the C-terminus of ApcEΔ or inserted between residues 76 and 78, thereby replacing the native hydrophobic loop domain. We describe the assembly, spectral characteristics, and intramolecular excitation energy transfer of two unique systems: in the first, the short-wavelength absorbing zinc-mesoporphyrin is bound to the HP7 domain and serves as an excitation-energy donor to the long-wavelength absorbing phycocyanobilin bound to the ApcE domain; in the second, the short-wavelength absorbing phycoerythrobilin is bound to the ApcE domain and serves as an excitation energy donor to the long-wavelength absorbing zinc-bacteriochlorophyllide bound to the HP7 domain. All the systems that were constructed and tested exhibited significant intramolecular fluorescence resonance energy transfer with yields ranging from 21% to 50%. This confirms that our modular, covalent approach for studying EET between the cyclic and open chain tetrapyrroles is reasonable, and may be extended to larger structures mimicking light-harvesting in cyanobacteria. The design, construction, and characterization process demonstrated many of the advances in constructing such model systems, particularly in our ability to control the fold and aggregation state of protein-based systems. At the same time, it underlines the potential of exploiting the versatility and flexibility of protein-based systems in assembling multiple pigments into effective light-harvesting arrays and tuning the spectral properties of multichromophore systems.

Roles of manganese in photosystem II dynamics to irradiations and temperatures

AbstactThe most amazing chemistry is the light-driven water splitting reaction occurred in the oxygen-evolving complex of phototsystem II in higher plants, green algae, and cyanobacteria. Mn, in the form of Mn4CaO5 cluster in photosystem II, is responsible for the catalytic water splitting reaction as well as plays roles in photosystem II dynamics to irradiation and temperatures. Manganese hypothesis of UV-initiated photoinhibition as a direct target is established, and thermal inactivation of photosystem II involves the valence and structural changes of manganese. Recent progresses in understanding the roles of manganese in photoinhibition especially under UV light and in thermal inactivation including elevated temperatures using synthetic models and native PS II complexes are summarized and evaluated. Potential problems and possible solutions are discussed and presented.

Current challenges in photosynthesis: from natural to artificial

Photosynthesis is a process by which plants, algae, cyanobacteria, and anoxygenic photosynthetic bacteria capture and store solar energy on a massive scale, in particular via the water-splitting chemistry (Hoganson and Babcock, 1997; Blankenship, 2002; Ferreira et al., 2004; Loll et al., 2005; Yano et al., 2006; Umena et al., 2011). It is the most important reaction on Earth, estimated to produce more than 100 billion tons of dry biomass annually; this means that photosynthesis is producing biomass equal to two Egyptian pyramids per hour. But, this will not be enough to sustain life on Earth by the year 2050. The global fossil fuels on which we currently depend are derived from millions of years of past photosynthetic activity. The fossil energy fuels are limited and must be replaced by renewable and environment-friendly energy source to support and sustain life on Earth (Lewis and Nocera, 2006; Blankenship et al., 2011). To address this immediate energy crisis, worldwide efforts are being made on artificial photosynthesis using the principles and mechanisms observed in nature (Brimblecombe et al., 2009; McConnell et al., 2010; Kanady et al., 2011; Wiechen et al., 2012; Najafpour et al., 2013). It is not a matter of mimicking natural photosynthesis, but to use its current knowledge to improve photosynthesis itself, as well as to produce biofuels, including hydrogen evolution by artificial means (Barber, 2009; Hou, 2010; Nocera, 2012; He et al., 2013). This book contains 10 chapters and presents recent advances in photosynthesis and artificial photosynthesis. It starts with two opinion articles on possible strategies to improve photosynthesis in plants and fascinating mechanisms of unidirectional photodamage of pheophytin in photosynthesis. The idea that plant photosynthesis is maximized due to the perfect evolution might be faulty. Leister evaluated and argued the issue openly and proposed that improvement of photosynthesis can be made by synthetic biology including genetic engineering, redesign or de novo creation of entire photosystems as well as conventional breeding (Leister, 2012). Unidirectional photodamage of a pheophytin molecule in photosystem II and purple bacterial reaction centers was observed. The mysterious phenomena were analyzed and discussed in terms of different possible functions of the pheophytin in photosynthesis (Hou, 2014). The book is followed by four review articles that discuss the current state of research on: photosynthetic water oxidation in natural and artificial photosynthesis, as obtained by mass spectrometry (MS) and Fourier transform infrared spectroscopy (FTIR); functional models of thylakoid lumen; and horizontal gene transfer in photosynthetic eukaryotes. The time-resolved isotope-ratio membrane-inlet mass spectrometry (TR-IR-MIMS) is able to determine the isotopic composition of gaseous products. Shevela et al briefly introduced the key aspects of the methodology, summarized the recent results on the mechanisms and pathways of oxygen formation in PS II using this unique technique and outlined the future perspectives of the application in water splitting chemistry (Shevela and Messinger, 2013) Another unique technique in probing the mechanism of water oxidation in PS II is the light-induced FTIR difference spectroscopy. Chu reviewed the recent fruitful structural data, and believed that the FTIR will continue to provide vital structural and mechanistic insights into the water-splitting process in PS II together with isotopic labeling, site-directed mutagenesis, model compound studies, and computational calculation (Chu, 2013). The thylakoid lumen offers the environment for oxygen evolution, electron transfer, and photoprotection in photosynthesis. Jarvi et al evaluated the recent studies of many lumen proteins and highlighted the importance of the thiol-disulfide modulation in controlling the functions of the thylakoid lumen proteins and their pathways of photosynthesis (Jarvi et al., 2013). Qiu et al discussed that importance of the horizontal gene transfer (HGT) in enriching the algal genomes and proposed that the alga endosymbionts may be the HGT vectors in photosynthetic eukaryotes (Qiu et al., 2013). Finally, the book offers four research articles, which focus on FTIR studies on photosynthetic reaction centers, functions of thylakoid protein kinases STN7 and STN8, photosynthesis acclimation of maize seedlings, and characterization of the newly discovered chlorophyll f-containing cyanobacterium Halomicronema hongdechloris. The computational calculation (ONIOM) is increasingly critical in interpreting the FTIR data in elucidating the structural and functional relationship in photosynthesis. Zhao et al using ONIOM type calculation to simulate isotope edited FTIR difference spectra for reaction centers with a variety of foreign quinones in the QA site and allows a direct assessment of the appropriateness of previous IR assignments and suggestions (Zhao et al., 2013). The protein kinases STN7 and STN8 are predominately responsible for the thylakoid phosphorylation in PS II. Wunder et al reported the effects of the STN8 expression levels on the formation and modulation of thylakoid proteins and kinases (Wunder et al., 2013). Hirth et al assessed the photosynthetic acclimation responses of the C3 and C4 plants under simulated field light conditions (Hirth et al., 2013). Recently a chlorophyll f (Chl f) in cyanobacterium Halomincronema hongdechloris was identified and has the most red-shifted absorption peak of 707 nm in oxygenic photosynthesis (Chen et al., 2010), which may enhance the potential photosynthesis efficiency for solar fuel production. The Halomincronema hongdechloris was characterized upon the exposure to the different light, pH, salinity, temperature, and nutrition to achieve the optimizing growth culture conditions (Li et al., 2014). Due to the extremely limited time frame for collecting manuscripts and the strict deadline for publishing this book, several planned manuscripts by world leaders, who had agreed to contribute, are unfortunately not included in this book. Thus, the current book provides a snapshot of the latest work in photosynthesis research. To obtain complete information on the current progress in the field of photosynthesis, we highly recommend reviews and research articles, published in 2013, in two volumes of Photosynthesis Research (Allakhverdiev et al., 2013a,b) In conclusion, the book provides readers with some of the most recent and exciting breakthroughs from natural to artificial photosynthesis, discusses the potential limitations of the results, and addresses open questions in photosynthesis and energy research. It is written by 31 young active scientists and established leading experts from Australia, Finland, Germany, Sweden, Taiwan, and the United States. We hope that this book is able to provide novel and insightful information to readers and stimulate the future research endeavors in the photosynthesis community.

Listening to PS II: Enthalpy, entropy, and volume changes

Photosystem II, located in the thylakoid membranes of green plants, algae, and cyanobacteria, uses sunlight to split water into protons, electrons, and a dioxygen molecule. The mechanism of its electron transfers and oxygen evolution including the structure of the protein and rates of the S-state cycle has been extensively investigated. Substantial progress has been made; however, the thermodynamics of PS II electron transfer and of the oxygen cycle are poorly understood. Recent progress in thermodynamic measurements in photosynthesis provides novel insights on the enthalpic and entropic contribution to electron transfer in proteins. In this review the thermodynamic parameters including quantum yield, enthalpy, entropy, and volume changes of PS II photochemistry determined by photoacoustics and other laser techniques are summarized and evaluated. Light-driven volume changes via electrostriction are directly related to the photoreaction in PS II and thus can be a useful measurement of PS II activity and function. The enthalpy changes of the reactions observed can be directly measured by photoacoustics. The apparent reaction entropy can also be estimated when the free energy is known. Dissecting the free energy of a photoreaction into enthalpic and entropic components provides critical information about mechanisms of PS II function. Potential limitations and future direction of the study of the thermodynamics of PS II electron transfer and oxygen evolution are presented.

Methodology of pulsed photoacoustics and its application to probe photosystems and receptors.

We review recent advances in the methodology of pulsed time-resolved photoacoustics and its application to studies of photosynthetic reaction centers and membrane receptors such as the G protein-coupled receptor rhodopsin. The experimental parameters accessible to photoacoustics include molecular volume change and photoreaction enthalpy change. Light-driven volume change secondary to protein conformational changes or electrostriction is directly related to the photoreaction and thus can be a useful measurement of activity and function. The enthalpy changes of the photochemical reactions observed can be measured directly by photoacoustics. With the measurement of enthalpy change, the reaction entropy can also be calculated when free energy is known. Dissecting the free energy of a photoreaction into enthalpic and entropic components may provide critical information about photoactivation mechanisms of photosystems and photoreceptors. The potential limitations and future applications of time-resolved photoacoustics are also discussed.

Thermodynamics of charge separation of photosystem I in the menA and menB null mutants of Synechocystis sp. PCC 6803 determined by pulsed photoacoustics.

When the biosynthesis of phylloquinone is inhibited in Synechocystis sp. PCC 6803 by interrupting the menA or the menB gene, photosystem I (PS I) recruits plastoquinone-9 (A(P)) to occupy the A(1) sites. In PS I from the menA and menB null mutants, forward electron transfer from the quinone to the FeS clusters occurs approximately 1000 times slower than in wild-type PS I [Semenov, A. Yu., Vassiliev, I. R., van der Est, A., Mamedov, M. D., Zybailov, B., Shen, G., Stehlik, D., Diner, B. A., Chitnis, P. R., and Golbeck, J. H. (2000) J. Biol. Chem. 275, 23429-23438]. To investigate the effect on thermodynamics, the enthalpy and volume changes of charge separation in PS I in the menA and menB mutants were measured using pulsed time-resolved photoacoustics on the nanosecond and microsecond time scales. The observed thermodynamic data are the same for the menA and menB mutants. This is expected because the recruited quinone (A(P)) is the same in both mutants. The volume change of PS I from the mutants following charge separation on both time scales was -17 +/- 2 A(3), less than that of the wild type, -21 A(3). The quantum yield of charge separation was found to be slightly lower (85 +/- 9%) than that of wild-type PS I (96 +/- 10%). The observed reaction is assigned to the formation of P(700)(+)A(P)(-) from P(700)*A(P). An enthalpy change (DeltaH) of -0.69 +/- 0.07 eV was obtained for this reaction. In contrast, a larger enthalpy change -0.8 eV for the formation of P(700)(+)A(1)(-) from P(700)* and an apparent entropy change (TDeltaS, T = 25 degrees C) of -0.2 eV were obtained in wild-type PS I [Hou, H. J. M., and Mauzerall, D. (2006) J. Am. Chem. Soc. 128, 1580-1586]. Taking the free energy to be -0.70 eV in PS I of the mutants, the apparent entropy is close to zero in the mutants. Since the apparent entropy change for the overall reaction of the production of P(700)(+)F(A/B)(-) from P(700)* is very likely the same as that of the wild type, +0.35 eV, this implies that the reaction of P(700)(+)A(P)(-)F(A/B) --> P(700)(+)A(P)F(A/B)(-) in the mutants is almost completely entropy driven (DeltaG = -0.07 eV and TDeltaS = +0.40 eV). These results show that not only the kinetics but also the thermodynamics of electron transfer reactions in PS I are significantly affected by the recruitment of the foreign plastoquinone-9 into the A(1) site.

Gas Chromatography-Mass Spectrometric Analysis of Forensic DrugFlunitrazepam upon Exposure to UV Irradiation

The environment may affect the forensic drug evidences in crime scene and is able to produce impurities, which contain vital information for tracing their origin of manufacture and can be used to provide link in crime scene investigation. In this work the response of forensic drug flunitrazepam to the UV irradiation was investigated by gas chromatography-mass spectrometry. We found the substantial change in GC pattern involving multiple GC peaks, indicating the complex reaction of degradation in flunitrazepam upon UV irradiation. GCMS analysis identified one of the GC components to be amino-flunitrazepam. The new GC peaks and the GC pattern change may serve as “chemical signatures” of flunitrazepam. Such information may promote the identification of the forensic drug flunitrazepam as a “chemical fingerprint” in forensic sciences.